Wavelength beam combining laser systems utilizing lens roll for chief ray focusing

ABSTRACT

In various embodiments, a wavelength beam combining laser system includes a fast-axis collimation lens that is rotated with respect to a plurality of emitters in order to converge the emitted beams onto a dispersive element and/or reduce the size of the multi-wavelength output beam of the system.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/012,335, filed Jun. 14, 2014, the entiredisclosure of which is hereby incorporated herein by reference. Thisapplication is also a continuation-in-part of U.S. patent applicationSer. No. 13/686,974, filed Nov. 28, 2012, which claims the benefit ofand priority to U.S. Provisional Patent Application No. 61/601,763,filed Feb. 22, 2012, the entire disclosure of each of which is herebyincorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to laser systems,specifically wavelength beam combining laser systems with adjustableoptics that overlap individual emitter beams to form a combined beam.

BACKGROUND

High-power laser systems are utilized for a host of differentapplications, such as welding, cutting, drilling, and materialsprocessing. Such laser systems typically include a laser emitter, thelaser light from which is coupled into an optical fiber (or simply a“fiber”), and an optical system that focuses the laser light from thefiber onto the workpiece to be processed. The optical system istypically engineered to produce the highest-quality laser beam, or,equivalently, the beam with the lowest beam parameter product (BPP). TheBPP is the product of the laser beam's divergence angle (half-angle) andthe radius of the beam at its narrowest point (i.e., the beam waist, theminimum spot size). The BPP quantifies the quality of the laser beam andhow well it can be focused to a small spot, and is typically expressedin units of millimeter-milliradians (mm-mrad). A Gaussian beam has thelowest possible BPP, given by the wavelength of the laser light dividedby pi. The ratio of the BPP of an actual beam to that of an idealGaussian beam at the same wavelength is denoted M², or the “beam qualityfactor,” which is a wavelength-independent measure of beam quality, withthe “best” quality corresponding to the “lowest” beam quality factor of1.

Wavelength beam combining (WBC) is a technique for scaling the outputpower and brightness from laser diode bars, stacks of diode bars, orother lasers arranged in one- or two-dimensional array. WBC methods havebeen developed to combine beams along one or both dimensions of an arrayof emitters. Typical WBC systems include a plurality of emitters, suchas one or more diode bars, that are combined using a dispersive elementto form a multi-wavelength beam. Each emitter in the WBC systemindividually resonates, and is stabilized through wavelength-specificfeedback from a common partially reflecting output coupler that isfiltered by the dispersive element along a beam-combining dimension.Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed onFeb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat.No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107,filed on Mar. 7, 2011, the entire disclosure of each of which isincorporated by reference herein.

While a variety of WBC techniques have been utilized to form high-powerlasers for a host of different applications, many such techniquesinvolve complicated arrangements of optical elements for beammanipulation, and, depending on the locations of the various opticalelements in the optical train of the system, it may be difficult toobtain the desired beam quality factor of the final combined beam and/orto maintain a relatively small overall footprint of the laser system.Thus, there is a need for improved WBC systems and techniques ofcombining the outputs of different laser emitters into an output beamhaving a high beam quality factor (i.e., a beam quality factor as closeto unity as possible) and that result in relatively compact lasersystems.

SUMMARY

In accordance with embodiments of the present invention, wavelength beamcombining laser systems feature multiple emitters (or “beam emitters”),e.g., diode bars or the individual diode emitters of a diode bar, whichare combined using a dispersive element to form a multi-wavelength beam.Each emitter in the system individually resonates and is stabilizedthrough wavelength-specific feedback from a common partially reflectingoutput coupler that is filtered by the dispersive element (e.g., adiffraction grating, a dispersive prism, a grism (prism/grating), atransmission grating, or an Echelle grating) along the beam-combiningdimension. In this manner, laser systems in accordance with embodimentsof the present invention produce multi-wavelength output beams havinghigh brightness and high power.

In accordance with various embodiments of the present invention, thelaser system features a one-dimensional or two-dimensional array of beamemitters, as well as a fast-axis collimation (FAC) lens and a beamrotator (or “optical rotator,” or “beam twister,” “optical rotationsystem,” or “optical twister”) downstream of the FAC lens. (Herein,“downstream” or “optically downstream,” is utilized to indicate therelative placement of a second element that a light beam strikes afterencountering a first element, the first element being “upstream,” or“optically upstream” of the second element.) The FAC lens (itself onlyor along with the beam rotator) is rotated relative to the arraydimension of the beam emitters in order to attain chief ray focus of thebeams emitted by the beam emitters and, thus, to overlap the individualbeams at the dispersive element of the WBC system and minimizebeam-quality degradation in the fast diverging axis (or “fast axis,”each beam typically having a fast diverging axis and a slow divergingaxis). Achievement of chief ray focus of the beam emitters at an opticaldistance substantially equal to the optical distance between the beamemitters and the dispersive element results in beam quality in the fastaxis to be substantially equal to the beam quality of any one of thebeam emitters. (Herein, “optical distance” between two components is thedistance between two components that is actually traveled by lightbeams; the optical distance may be, but is not necessarily, equal to thephysical distance between two components due to, e.g., reflections frommirrors or other changes in propagation direction experienced by thelight traveling from one of the components to the other.)

Like the focusing optics (i.e., one or more lenses) in conventional WBClaser systems, embodiments of the invention achieve chief ray focus viarotation of a FAC lens downstream of the beam emitters, with or withoutrotation of a beam rotator downstream of the FAC lens, even in theabsence of additional focusing optics. The rotation of the FAC lensgenerates a linear pointing effect across the beam emitters in the fastaxis, in which fast-axis pointing changes linearly from one emitter tothe next across the entire array of beam emitters. The beam rotator alsorotates the fast and slow axes of the beams, typically by approximately90°.

Embodiments of the present invention may also reduce the footprint ofthe laser system, increase output power of the output beam, and/orenable the brightness of the output beam to be adjusted (e.g., in realtime and/or via feedback control). Through the various embodiments andtechniques described herein a reduced-size, stabilized, variablebrightness multi-wavelength output laser system may be achieved.

The approaches and embodiments described herein may apply to one- andtwo-dimensional beam combining systems along the slow axis, fast axis,or other beam combining dimension. In addition, the techniques may applyto external and non-external cavity wavelength beam combining systems.

Embodiments of the present invention may be utilized to couple the oneor more input laser beams into an optical fiber. In various embodiments,the optical fiber has multiple cladding layers surrounding a singlecore, multiple discrete core regions (or “cores”) within a singlecladding layer, or multiple cores surrounded by multiple claddinglayers.

Herein, “optical elements” may refer to any of lenses, mirrors, prisms,gratings, and the like, which redirect, reflect, bend, or in any othermanner optically manipulate electromagnetic radiation. Herein, beamemitters, emitters, or laser emitters, or lasers include anyelectromagnetic beam-generating device such as semiconductor elements,which generate an electromagnetic beam, but may or may not beself-resonating. These also include fiber lasers, disk lasers, non-solidstate lasers, vertical cavity surface emitting lasers (VCSELs), etc.Generally, each emitter includes a back reflective surface, at least oneoptical gain medium, and a front reflective surface. The optical gainmedium increases the gain of electromagnetic radiation that is notlimited to any particular portion of the electromagnetic spectrum, butthat may be visible, infrared, and/or ultraviolet light. An emitter mayinclude or consist essentially of multiple beam emitters such as a diodebar configured to emit multiple beams (i.e., each diode in the bar emitsa single beam).

Laser diode arrays, bars and/or stacks, such as those described in thefollowing general description may be used in association withembodiments of the innovations described herein. Laser diodes may bepackaged individually or in groups, generally in one-dimensionalrows/arrays (diode bars) or two dimensional arrays (diode-bar stacks). Adiode array stack is generally a vertical stack of diode bars. Laserdiode bars or arrays generally achieve substantially higher power, andcost effectiveness than an equivalent single broad area diode.High-power diode bars generally contain an array of broad-area emitters,generating tens of watts with relatively poor beam quality; despite thehigher power, the brightness is often lower than that of a broad arealaser diode. High-power diode bars may be stacked to produce high-powerstacked diode bars for generation of extremely high powers of hundredsor thousands of watts. Laser diode arrays may be configured to emit abeam into free space or into a fiber. Fiber-coupled diode-laser arraysmay be conveniently used as a pumping source for fiber lasers and fiberamplifiers.

A diode-laser bar is a type of semiconductor laser containing aone-dimensional array of broad-area emitters or alternatively containingsub arrays containing, e.g., 10-20 narrow stripe emitters. A broad-areadiode bar typically contains, for example, 19-49 emitters, each havingdimensions on the order of, e.g., 1 μm×100 μm. The beam quality alongthe 1 μm dimension or fast-axis is typically diffraction-limited. Thebeam quality along the 100 μm dimension or slow-axis or array dimensionis typically many times diffraction-limited. Typically, a diode bar forcommercial applications has a laser resonator length of the order of 1to 4 mm, is about 10 mm wide and generates tens of watts of outputpower. Most diode bars operate in the wavelength region from 780 to 1070nm, with the wavelengths of 808 nm (for pumping neodymium lasers) and940 nm (for pumping Yb:YAG) being most prominent. The wavelength rangeof 915-976 nm is used for pumping erbium-doped or ytterbium-dopedhigh-power fiber lasers and amplifiers.

A diode stack is simply an arrangement of multiple diode bars that candeliver very high output power. Also called diode laser stack, multi-barmodule, or two-dimensional laser array, the most common diode stackarrangement is that of a vertical stack which is effectively atwo-dimensional array of edge emitters. Such a stack may be fabricatedby attaching diode bars to thin heat sinks and stacking these assembliesso as to obtain a periodic array of diode bars and heat sinks. There arealso horizontal diode stacks, and two-dimensional stacks. For the highbeam quality, the diode bars generally should be as close to each otheras possible. On the other hand, efficient cooling requires some minimumthickness of the heat sinks mounted between the bars. This tradeoff ofdiode bar spacing results in beam quality of a diode stack in thevertical direction (and subsequently its brightness) is much lower thanthat of a single diode bar. There are, however, several techniques forsignificantly mitigating this problem, e.g., by spatial interleaving ofthe outputs of different diode stacks, by polarization coupling, or bywavelength multiplexing. Various types of high-power beam shapers andrelated devices have been developed for such purposes. Diode stacks mayprovide extremely high output powers (e.g. hundreds or thousands ofwatts).

In an aspect, embodiments of the invention feature a laser system thatincludes or consists essentially of a plurality of beam emitters, afast-axis collimation (FAC) lens, a beam rotator, a dispersive element,and a partially reflective output coupler. Each beam emitter emits anindividual beam that has a fast-diverging axis and a slow-divergingaxis. The plurality of beam emitters are arranged in an array having alateral dimension. The FAC lens is disposed optically downstream of theplurality of beam emitters, receives the emitted beams, and collimatesthe beams along their fast-diverging axes. The FAC lens has a lateraldimension rotated at a non-zero angle with respect to the lateraldimension of the plurality of beam emitters. The beam rotator isdisposed optically downstream of the FAC lens, rotates the beams, anddirects the rotated beams toward a dispersive element. The dispersiveelement receives and disperses the rotated beams. The rotation of theFAC lens with respect to the plurality of beam emitters may cause therotated beams to at least partially (e.g., substantially completely)overlap at the dispersive element. The partially reflective outputcoupler receives the dispersed beams, reflects a first portion thereofback toward the dispersive element, and transmits a second portionthereof as a multi-wavelength output beam. The laser system may lackfocusing optics optically downstream of the beam rotator and opticallyupstream of the dispersive element.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The beam rotator may flip thefast-diverging axis and the slow-diverging axis of each beam (i.e.,rotate each beam by approximately 90°). The FAC lens may be discrete andseparate from the beam rotator. The FAC lens and the beam rotator may beportions of a unitary (i.e., one-piece) optical element. The beamrotator may have a lateral dimension rotated at the non-zero angle withrespect to the lateral dimension of the plurality of beam emitters. Thebeam rotator may have a lateral dimension rotated at a second non-zeroangle with respect to the lateral dimension of the plurality of beamemitters. The second non-zero angle may not be equal to the non-zeroangle. The beam rotator may have a lateral dimension that is not rotatedwith respect to the lateral dimension of the plurality of beam emitters.The dispersive element may include or consist essentially of adiffraction grating (e.g., a transmissive diffraction grating or areflective diffraction grating).

In another aspect, embodiments of the invention feature a method forconfiguring a laser system. A plurality of beam emitters is provided.Each beam emitter emits an individual beam that has a fast-divergingaxis and a slow-diverging axis. The plurality of beam emitters arearranged in an array having a lateral dimension. A fast-axis collimation(FAC) lens disposed optically downstream of the plurality of beamemitters is provided. The FAC lens has a lateral dimension. The FAC lensreceives the emitted beams and collimates the beams along theirfast-diverging axes. A beam rotator disposed optically downstream of theFAC lens is provided. The beam rotator rotates the beams and directs therotated beams toward a dispersive element. A dispersive element forreceiving and dispersing the rotated beams is provided. A partiallyreflective output coupler is provided. The partially reflective outputcoupler receives the dispersed beams, reflects a first portion thereofback toward the dispersive element, and transmits a second portionthereof as a multi-wavelength output beam having a beam size. Thelateral dimension of the FAC lens is rotated with respect to the lateraldimension of the plurality of beam emitters to reduce the beam size ofthe output beam.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The lateral dimension of the FAC lensmay be rotated with respect to the lateral dimension of the plurality ofbeam emitters until the beam size is substantially minimized. Therelative rotation of the FAC lens and the plurality of beam emitters maybe performed by rotating the FAC lens, rotating the plurality of beamemitters, or both. The rotation of the lateral dimension of the FAC lenswith respect to the lateral dimension of the plurality of beam emittersmay cause the rotated beams to at least partially overlap (e.g.,substantially completely overlap) at the dispersive element. The beamrotator may have a lateral dimension. The lateral dimension of the beamrotator may be rotated with respect to the lateral dimension of theplurality of beam emitters. The lateral dimensions of the beam rotatorand the FAC lens may be rotated by approximately the same angle withrespect to the lateral dimension of the plurality of beam emitters. Thelateral dimensions of the beam rotator and the FAC lens may be rotatedsubstantially simultaneously. The beam rotator may flip thefast-diverging axis and the slow-diverging axis of each beam. The FAClens may be discrete and separate from the beam rotator. The FAC lensand the beam rotator may be portions of a unitary optical element. Thedispersive element may include or consist essentially of a diffractiongrating (e.g., a transmissive diffraction grating or a reflectivediffraction grating).

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “substantially” and “approximately” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. Herein, the terms “radiation” and“light” are utilized interchangeably unless otherwise indicated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a schematic illustration of a conventional wavelength beamcombining (WBC) laser system along a non-beam-combining dimension;

FIG. 1B is a schematic illustration of the WBC laser system of FIG. 1Aalong the beam-combining dimension;

FIGS. 2A-2C are schematic illustrations of WBC laser systems withnon-confocal focusing optics in accordance with embodiments of theinvention;

FIG. 3 is a schematic illustration of a WBC laser system lackingfocusing optics upstream of the dispersive element in accordance withembodiments of the invention;

FIG. 4 is a schematic illustration of a WBC laser system utilizing acurved dispersive element in accordance with embodiments of theinvention;

FIGS. 5A-5C are schematic illustrations of WBC laser systemsincorporating spatial combining optical systems configured tosubstantially overlap wavelength feedback along a non-beam-combiningdimension in accordance with embodiments of the invention;

FIG. 6 is a graph of normalized beam quality as a function of theposition of the dispersive element for an exemplary WBC laser system ofFIG. 2B in accordance with embodiments of the invention;

FIGS. 7A and 7B are schematic illustrations of beams from WBC systemsin-coupled into optical fibers in accordance with embodiments of theinvention;

FIGS. 8A and 8B are schematic illustrations of beams from WBC systemsbeing coupled into optical fibers with adjustable and/or variable beamquality in accordance with embodiments of the invention;

FIG. 9A is a head-on schematic of fast-axis collimation lens rotationwith respect to an array of beam emitters in accordance with embodimentsof the invention;

FIG. 9B is a lateral schematic of effective positions of the fast-axiscollimation lens and one of the beam emitters of FIG. 9A before andafter rotation in accordance with embodiments of the invention; and

FIG. 9C is a schematic illustration of a WBC laser system incorporatinga rotatable fast-axis collimation lens to converge each individual beamemitted by an array of beam emitters in accordance with embodiments ofthe invention.

DETAILED DESCRIPTION

FIGS. 1A and 1B illustrate a conventional external-cavityone-dimensional wavelength beam combining (WBC) laser system along anon-beam-combining dimension 130 (FIG. 1A) and along a beam-combiningdimension 140 (FIG. 1B). As shown, such systems may include aone-dimensional diode bar 102 having a back reflective surface 104, again medium 106 with two or more diode emitters 105, a front reflectivesurface 108, a combining optic (or “focusing lens”) 110, a dispersiveelement 112, and a partially reflecting output coupler 114. Thecombining optic 110 is typically placed an optical distance 120 a awayfrom the front reflective surface 108 of the diode bar 102, while thedispersive element 112 is placed an optical distance 120 b away fromcombining optic 110, where both optical distances 120 a, 120 b aresubstantially equal to the focal length of the combining optic 110. Theoutput coupler 114 is spaced away from the dispersive element 112 andreflects a portion 116 of the generated beams (feedback) to thedispersive element 112 and transmits a multi-wavelength output beam 118that includes the wavelengths emitted by the various emitters 105. Thecombining lens 110 is typically placed to accomplish two functions. Thefirst function is to overlap all the chief rays from all the diodeelements onto the dispersive element 112. The second function is tocollimate each beam in both axes.

However, more compact WBC systems, such as those shown in FIGS. 2A and2B, may be achieved by intentionally placing the diode bar 102 ordispersive element 112 at a position other than the focal plane of thecombining optical element 110. If the combining optical element 110 isplaced less than a focal length from the diode bar 102, than anadditional collimating optic(s) 124 may be placed before or after thedispersive element 112 and before the partially reflective outputcoupler 114 as shown in FIG. 2A. This allows the optical path lengthbetween the diode bar and output coupler to be reduced by almost (orsubstantially) a full focal length of combining element 110, for examplewhen combining element 110 is placed adjacent to the front surface/facet108 of diode bar 102.

In a variation of this embodiment, collimating optic(s) 124 a may beindividually placed in front of each emission point along the frontsurface/facet of the diode bar and before the combining optical element110, which still results in a more compact WBC system. In thisvariation, the collimating optic(s) 124 a may include or consistessentially of an array of micro-optical fast-axis collimating (FAC)lenses, slow-axis collimating lenses (SAC) or combination of both.Collimating each beam helps to ensure that proper wavelengthstabilization feedback is received into each of the diode elements. Thishelps each diode element produce a unique wavelength that is stabilizedand less susceptible to shifting, and thus a multi-wavelength outputbeam profile of high brightness and high power may be achieved.

As shown in FIG. 2A, the dispersive element (e.g., a diffractiongrating) is placed substantially at the back focal plane of the focusinglens. As shown, to a first approximation, the lens with focal length 190only converges the chief rays for each of the diode elements. This maybe understood from the Gaussian beam transformation by a lens equation1/(s+zr²/(s−f))+1/s″=1/f, where s and s″ are the input and output waistlocations, zr is the Raleigh range, and f is the focal length. Thus, thechief rays 160 are overlapping at the grating while each beam is stilldiverging and thus forming diverging rays (shown as dashed lines). Thediverging rays may or may not be later collimated by an optical element,such as optical element 124. With all the diode element beams overlappedon the dispersive element 112, the output beam quality is generally thatof a single emitter. Again, one advantage of this system is that thesize may be considerably smaller, as it does not require an opticalspacing between diode elements and the dispersive element equal to twicethe focal length of the lens 110. In some embodiments, the beam path isreduced by almost half or more. The spacing between various elements asdescribed herein may be slightly longer, equal to, or slightly shorterthan the focal length 190.

Alternatively, an embodiment devoid of collimating optic(s) 124 isillustrated in FIG. 2B. Combining optical element 110 is placed a focallength from the front facet 108 and as a result collimates the lightfrom each diode element. A reduced system size is still achieved byplacing dispersive element 112 less than a focal length from combiningoptical element 110. The brightness of the multi-wavelength beam isstill increased as compared to the initial array of beams produced bydiode bar 102.

As shown in FIG. 2B, the dispersive element 112, is placed much closerthan the back focal plane. However, the penalty of such a system isthere is some degradation in the output beam quality. To furtherillustrate, in one variation of this embodiment, it is assumed that thediode elements 102 are a single 10-mm wide bar with 47 emitters. Eachemitter may have a FAC lens (not shown) and no SAC lens. Inclusion of aSAC lens does not change the exemplary results detailed here. The focallength of the FAC lens in this variation is 910 μm. In this variation,the diode bar is operating at a 1 μm wavelength. With each beam beingdiffraction limited along the fast axis, the typical full divergenceafter the FAC lens is about 1 milliradian (mrd). Along the slow-axis thebeam is diverging about 100 mrd. We assume that the combining opticalelement 110 or transform lens has a focal length of 150 mm. The outputbeam quality is approximately M²=(θ×π/(4×λ))×sqrt((z×x/f)²+1), where λ=1μm, z is the distance after the lens to the grating and center at theback focal plane, x is the dimension of the array (e.g., 10 mm), and θis the individual beam divergence after the grating.

FIG. 6 is a graph illustrating the approximate output beam quality as afunction of grating position. The output beam quality is normalized tothe ideal case where the grating is at the back focal plane of theoptics. As expected at z=0 or the back focal plane the normalized beamquality is 1 and grows to about 33 times at z=500. The normalizedfree-running beam quality is about M²˜47/ff=47/0.5=94, where ff is thenear field fill-factor of the diode emitter. Thus, even at z=500, thebeam quality of the system is still better than free a running system,one without WBC, by about three times. In FIGS. 1A and 1B, the combiningelement 110 also acted as the collimating element. In FIG. 2A thefunction of the combining element 110 is primarily to focus the chiefrays 160 onto the dispersive element 112 and an additional collimatingelement 124 is placed after the dispersive element 112 generally at thefocal plane of 124 to collimate the diverging rays 162. In FIG. 2B,combining element 110 is placed at approximately a focal length from thefront aperture 108 and collimates the diverging rays, but because of theshortened distance from combining element 110 to dispersive element 112,the chief rays do not completely overlap onto each other as in typicalconventional WBC arrangements. The multi-beam output still has anincrease in brightness, but as suggested by FIG. 6 is not at optimalbrightness.

FIG. 2C illustrates a WBC system that enables a shortened beam pathwayand substantially separates the functionality of combining chief raysand collimating diverging rays into two separate optical elements (orsystems) positioned before the dispersive element. A combining element210 is positioned at a distance substantially less than its respectivefocal length F1 away from the front aperture 108 on one side andapproximately a focal length F1 away from the dispersive element 112 onthe other side. This allows combining element 210 to direct the chiefrays from each diode emitter of diode bar 102 to overlap orsubstantially overlap on the dispersive element 112. At the same time, acollimating optical element 224 is placed approximately a focal distanceF2 away from the front aperture of each diode emitter on one side and ata distance less than focal length F2 from the dispersive element on theother side. Similarly, the primary function of the collimating opticalelement 224 is to collimate the diverging rays. One skilled in the artwill readily acknowledge that both optical elements 210, 224 haveoptical power along the same dimension and as a result will have someeffect on the actual placement of each optical element with respect tothe front aperture and dispersive element. However, this interdependencyis managed in-part by the placement of each optical elementsubstantially close to the front aperture on one side and the dispersiveelement on the other side. Thus, the combining optical element 210primarily dominates the combining of the chief rays on the dispersiveelement 112, but is influenced by the prescription of collimatingoptical element 224 and vice versa.

Other embodiments described herein also reduce system size and even theneed for an optical combining element(s) through using alternativeposition-to-angle methods. For example, FIG. 3 illustrates a WBC systemdevoid of an optical combining element. Each diode bar 102 (which insome cases may include or consist essentially of only a single diodeemitter) may be mechanically positioned in a manner that the chief rays160 exiting the diode bars 102 overlap at a common region on thedispersive element 112 as shown. (In other variations of thisembodiment, and similar to FIG. 2B, the beams do not completely overlapat the dispersive element, but the spatial distance between each along acombining dimension is reduced.) The diverging rays 162 (dashed lines),are later collimated by collimating optic(s) 124 positioned between thedispersive element 112 and the partially reflective output coupler 114.(Some variations of this embodiment include replacing collimating optic124 with individual FAC and/or SAC lenses positioned at the frontsurface or facet of each diode bar.) This embodiment thus increasesbrightness while reducing the number of optical elements required aswell as reducing overall system size.

In another embodiment shown in FIG. 4, a curved diffraction grating 412is placed a focal length F1 from the diode bar 102. The curveddiffraction grating combines the emitted beams into a multi-wavelengthbeam that is transmitted to the partially-reflective output coupler 114,where a portion is reflected back towards the curved diffraction grating412. The wavelengths of the reflected beams are then filtered by thediffraction grating and transmitted back into each emitter of diode bar102, where each emitter is stabilized to a particular wavelength. Thelimitation of brightness in this type of system generally hinges on theamount of power the curved diffraction grating can handle. Thisembodiment illustrates an optical architecture reducing the number ofoptical elements and shortening the beam path while increasing thebrightness of a multi-wavelength output beam. Any degradation of thebeam quality results as a function of the width 475 over the entiredistance of the beam profile 485.

FIGS. 5A-5C illustrate various embodiments of spatial combinersincorporated along the non-beam combining dimension 130 of a WBC systemthat help to increase the amount of system output power that may becoupled into a fiber (e.g., as illustrated in FIGS. 7A and 7B). FIG. 5Aillustrates a simple black box model, showing a spatial combiner 550that may be configured in various ways. The key concepts are to overlapfeedback for each beam along its original pathway along thenon-combining dimension to be received back into each of the originalemission points or emitters. The feedback is often the result of thepartially-reflective output coupler 114, which reflects a portion of themulti-wavelength beams. For example, two optical pathways 515, as shownin FIG. 5A, are where beams or multiple beams of radiation travelthrough the spatial combiner 550, which helps to overlap the radiationreflected by output coupler 114, back into the emitters 505, thusforming a stabilized system.

FIG. 5B illustrates one embodiment of a system utilizing a spatialcombiner in which multiple lenses 555 image the beam waist of eachemitter (or array of emitters) from each diode bar 102 onto or near thepartially-reflective output coupler 114. This helps the two-dimensionalarray of emitters (here shown as two diode bars having at least oneemitter each) from diverging too quickly, properly guiding feedback intothe original emitters, thus stabilizing each emitter along the non-beamcombining dimension and allowing the multi-wavelength output to be in acompact form. FIG. 5C illustrates an embodiment having a spatialcombiner 550 c that uses a lens 555 and an afocal telescoping system(565 a, 565 b) to keep the reflected beams overlapping the originalpathways and thus stabilizing each of the emitters.

FIGS. 7A and 7B illustrate an effect of spatial beam combining on fibercoupling. As shown in FIG. 7A, an optical fiber 701 a has a cladding 705and core 707 configured to receive a multi-wavelength beam 709.Multi-wavelength beam 709 is usually formed by combining single row (onedimension) of diode emitters (often generated by a single diode bar)into the size of a single element configured to be received by the core707. However, in two-dimensional or multi-row/array diode emittersconfigurations (which may include, e.g., multiple diode bars) a spatialcombiner, such as those described in FIGS. 5A-5C, may be used toconfigure a multi-wavelength beam profile that has the size of threeoriginal elements/emitters stacked in a single column three into asimilar sized core 707 of an optical fiber 701 b. For example, one barmay include or consist essentially of 49 emitters that are reduced toapproximately the size of one emitter. Stacking or arranging opticallyor mechanically three bars with 49 emitters each a profile 709 a,b,c(709 a, b and c each representing a row or diode bar) may produce amulti-wavelength beam configured to be received into an optical fiberthat increases the brightness and power by a factor of three times.

As described above, various spacing between the diode emitters,combining optical element and dispersive element have been discussed(including those systems devoid of an optical combining element). Alsodisclosed in FIG. 6 was a graph illustrating the normalized beam qualitymeasured in terms of beam parameter product (BPP). The BPP of eachsystem described herein may adjusted from a low BPP to a higher BPP. Inlaser manufacturing, cutting, and welding, various materials, thickness,type of cuts, etc. may require a laser to have a flexible output basedon BPP. Some materials require the highest quality beam output (orlowest BPP available) to cut; however, in other circumstances where theparameters of the material, cut, and/or thickness have been altered, thesame high quality may be insufficient to efficiently perform the desiredtask. Thus, the WBC system may be intentionally adjusted to reduce beamquality in order to more effectively accomplish a particularmanufacturing task. Therefore, in embodiments of the invention,adjustable positioning of diode emitters with respect to the dispersiveelement, diode emitters with respect to the combining optical element,combining optical element with respect to the dispersive element, helpto create a flexible WBC laser system that may accomplish a greaterrange of tasks. These adjustable position configurations may includemanual and real-time adjustments, e.g., thin metal applications andthick metal applications, as well as the type of metal to be cut, whichsometimes require different cutting parameters relative to beam quality.In another embodiment, in addition to linearly positioning the beams andelements along the beam combining dimension, the angular position of atleast one of the beams is repositionable. This repositioning may beautomated that it may occur in real time. One way of accomplishing thisis by placing a rotatable optical element, such as a piece of glass,after the multi-wavelength output of the WBC system and prior to a fiberoptical mount (FOM).

FIGS. 8A and 8B illustrate an embodiment incorporating a rotatableoptical element 820 (or beam path adjuster) in the optical pathway 815containing the multi-wavelength output beam from WBC system 810 to a FOM825 configured to direct the multi-wavelength beam into fiber 830 withits associated numerical aperture (NA) and width. FIG. 8A illustrates aconfiguration in which the rotatable optical element 820 has a surfacenormal to the optical pathway 815, thus allowing the optical pathway tocontinue on to the FOM 825 with no or minimal interference.Alternatively, rotatable optical element 820 may be positioned so thatthe surface is offset at a non-normal angle, causing themulti-wavelength beam to be redirected to a point 822 and thus follow anew optical path 817 that is offset by a distance 824 from an unalteredoptical pathway 819. This causes an angle 826 at which themulti-wavelength beam enters fiber 830 from FOM 825 to increase. Thisincrease in angle reduces the quality of the multi-wavelength beam(i.e., increases its BPP). As stated, in some instances a less brightbeam is actually more desirable for particular applications, thus havinga tunable or adjustable WBC system may be configured to accomplishvarious tasks at optimal settings for those particular tasks. In someembodiments, the rotatable optical element 820 may be manually adjustedwhile in others the rotation process may be automated. In yet otherconfigurations in which the output beam quality of the system isadjustable, the output power of each emitter may adjusted fromcompletely off to full power output.

As mentioned above, WBC laser systems in accordance with variousembodiments of the present invention feature a FAC lens and a beamrotator downstream of the array of beam emitters. Rotation of the FAClens (itself only or along with the beam rotator) relative to the arraydimension of the beam emitters enables chief ray focus of the beamsemitted by the beam emitters and, thus, overlap of the individual beamsat the dispersive element. (Herein, relative rotation of the beamemitters and the FAC lens is understood to include rotation of the arrayof beam emitters, rotation of the FAC lens, or both, so long as there isan angle of rotation introduced therebetween.) Achievement of chief rayfocus of the beam emitters at an optical distance substantially equal tothe optical distance between the beam emitters and the dispersiveelement results in beam quality in the fast axis to be substantiallyequal to the beam quality of any one of the beam emitters. FIGS. 9A and9B illustrate the impact of rotation of a downstream FAC lens on emittedbeams. FIG. 9A is a head-on view facing the emission face of an array ofbeam emitters 105, each of which is designated by a horizontal line.Rotation of the FAC lens from an unrotated configuration 900 (designatedby the solid outline) to a rotated configuration 905 (designated by thedashed outline) induces fast-axis pointing (i.e., variation in theorientation of the fast axis, designated by the vertical arrows) in theindividual beams. As shown in FIG. 9B, which is a side view of anindividual emitter 105 and a FAC lens, the magnitude of the fast-axispointing is generally a function of the change in relative positionbetween the FAC lens and the emitter 105 when the FAC lens is rotatedfrom configuration 900 to configuration 905. As shown, the relativechange in vertical position of the FAC lens results in fast-axispointing of an angle 920.

FIG. 9C depicts a WBC system 920 that incorporates an array of emitters105, a FAC lens 925, and a beam rotator 930. FIG. 9C depicts WBC system920 as including five emitters 105, but embodiments of the invention mayinclude more or fewer than five emitters 105. For example, the emitters105 may be individual diodes in a diode bar or in an array of diodebars. The beam rotator 930 rotates the fast and slow axes of the beams(by, e.g., approximately 90°, thereby flipping the fast and slow axes)emitted by emitters 105, and beam rotator 930 may include or consistessentially of, e.g., two spaced-apart cylindrical lenses or a unitaryoptical element having the same beam-rotation properties as twospaced-apart cylindrical lenses. The FAC lens 925 (and, in someembodiments, the beam rotator 930) is rotatable with respect to thearray of beam emitters 105 as shown in FIG. 9A. In fact, in theconfiguration pictured in FIG. 9C, at least the FAC lens 925 is rotatedwith respect to the array of beam emitters 105, as the beams emitted bythe beam emitters 105 are converging onto (and substantially overlappingeach other at) the dispersive element 112 once the beams propagatethrough the FAC lens 925 and the beam rotator 930. The dispersiveelement 112 disperses the beams, a portion of which is reflected by thepartially reflective output coupler 114 back to the emitters 105, and asecond portion of which passes through the partially reflective outputcoupler 114 as multi-wavelength output beam 118. FIG. 9C also depicts,in dashed lines, the paths upon which the beams would propagate absentrotation of the FAC lens 925.

As mentioned herein, in various embodiments of the invention the FAClens 925 is rotated with respect to the emitters 105 while the beamrotator 930 is not. In other embodiments, both the FAC lens 925 and thebeam rotator 930 are rotated with respect to the array of emitters 105(e.g., by approximately the same angle with respect to the emitters105). In various embodiments, the FAC lens 925 and the beam rotator 930are portions of a single unitary optical element that is rotated withrespect to the emitters 105 such that the beams converge at thedispersive element 112. Although FIG. 9C depicts the beams from emitters105 converging to and overlapping upon the dispersive element 112without any additional focusing optics disposed optically between thebeam rotator 930 and the dispersive element 112, in various embodimentsthe FAC lens 925 (and, in some embodiments, the beam rotator 930) isrotated with respect to the emitters 105, but the angle of rotation isinsufficient to achieve complete overlap of the beams at the dispersiveelement 112 (for example, the optical distance between the beam rotator930 and the dispersive element 112 may be insufficient to achievesubstantially complete beam overlap). In such embodiments, additionalfocusing optics may be disposed optically between the beam rotator 930and the dispersive element 112 in order to provide further focusing ofthe beams so that they overlap at the dispersive element 112. Suchfocusing optics may be disposed at an optical distance less than itsfocal length from the beam rotator 930 and/or the dispersive element112, e.g., as described herein and in U.S. patent application Ser. No.14/667,094, filed on Mar. 24, 2015, the entire disclosure of which isincorporated herein by reference.

In various embodiments of the invention, the angle of rotation of theFAC lens 925 (and, in some embodiments, of the beam rotator 930) isconfigured (e.g., during set-up of the system 920) via feedback from themultiple-wavelength output beam 118. In various embodiments, the WBCsystem 920 is set up as pictured in FIG. 9C with the FAC lens 925 (and,in some embodiments, the beam rotator 930) rotated at a first angle(which may be approximately 0°) with respect to the beam emitters 105.The fast-axis beam size at the output coupler 114 (e.g., of the outputbeam 118) is monitored, and the FAC lens 925 (and, in some embodiments,the beam rotator 930) is rotated to one or more angles different fromthe first angle until the fast-axis beam size at the output coupler 114is minimized. This minimum beam size indicates that the beams emitted byemitters 105 are substantially overlapping at the dispersive element112, and thus that degradation in the fast-axis beam quality isminimized or substantially eliminated. As mentioned above, the resultingbeam quality (e.g., along the fast axis) of the output beam 118 may besubstantially equal to the beam quality of a single one of the emitters105.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A laser system comprising: a plurality of beamemitters each emitting an individual beam, each beam having afast-diverging axis and a slow-diverging axis, the plurality of beamemitters being arranged in an array having a lateral dimension; disposedoptically downstream of the plurality of beam emitters, a fast-axiscollimation (FAC) lens (A) for (i) receiving the emitted beams and (ii)collimating the beams along their fast-diverging axes, and (B) having alateral dimension rotated at a non-zero angle with respect to thelateral dimension of the plurality of beam emitters; disposed opticallydownstream of the FAC lens, a beam rotator for (i) rotating the beamsand (ii) directing the rotated beams toward a dispersive element; adispersive element for receiving and dispersing the rotated beams,wherein the rotation of the FAC lens with respect to the plurality ofbeam emitters causes the rotated beams to at least partially overlap atthe dispersive element; and a partially reflective output coupler forreceiving the dispersed beams, reflecting a first portion thereof backtoward the dispersive element, and transmitting a second portion thereofas a multi-wavelength output beam.
 2. The laser system of claim 1,wherein the beam rotator flips the fast-diverging axis and theslow-diverging axis of each beam.
 3. The laser system of claim 1,wherein the FAC lens is discrete and separate from the beam rotator. 4.The laser system of claim 1, wherein the FAC lens and the beam rotatorare portions of a unitary optical element.
 5. The laser system of claim1, wherein the beam rotator has a lateral dimension rotated at thenon-zero angle with respect to the lateral dimension of the plurality ofbeam emitters.
 6. The laser system of claim 1, wherein the beam rotatorhas a lateral dimension rotated at a second non-zero angle with respectto the lateral dimension of the plurality of beam emitters, the secondnon-zero angle not being equal to the non-zero angle.
 7. The lasersystem of claim 1, wherein the beam rotator has a lateral dimension thatis not rotated with respect to the lateral dimension of the plurality ofbeam emitters.
 8. The laser system of claim 1, wherein the dispersiveelement comprises a diffraction grating.
 9. A method for configuring alaser system, the method comprising: providing a plurality of beamemitters each emitting an individual beam, each beam having afast-diverging axis and a slow-diverging axis, the plurality of beamemitters being arranged in an array having a lateral dimension;providing a fast-axis collimation (FAC) lens disposed opticallydownstream of the plurality of beam emitters, the FAC lens (A) for (i)receiving the emitted beams and (ii) collimating the beams along theirfast-diverging axes, and (B) having a lateral dimension; providing abeam rotator disposed optically downstream of the FAC lens, the beamrotator for (i) rotating the beams and (ii) directing the rotated beamstoward a dispersive element; providing a dispersive element forreceiving and dispersing the rotated beams; providing a partiallyreflective output coupler for receiving the dispersed beams, reflectinga first portion thereof back toward the dispersive element, andtransmitting a second portion thereof as a multi-wavelength output beamhaving a beam size; and rotating the lateral dimension of the FAC lenswith respect to the lateral dimension of the plurality of beam emittersto reduce the beam size of the output beam.
 10. The method of claim 9,wherein the lateral dimension of the FAC lens is rotated with respect tothe lateral dimension of the plurality of beam emitters until the beamsize is substantially minimized.
 11. The method of claim 9, wherein therotation of the lateral dimension of the FAC lens with respect to thelateral dimension of the plurality of beam emitters causes the rotatedbeams to at least partially overlap at the dispersive element.
 12. Themethod of claim 9, wherein the beam rotator has a lateral dimension, andfurther comprising rotating the lateral dimension of the beam rotatorwith respect to the lateral dimension of the plurality of beam emitters.13. The method of claim 12, wherein the lateral dimensions of the beamrotator and the FAC lens are rotated by approximately the same anglewith respect to the lateral dimension of the plurality of beam emitters.14. The method of claim 12, wherein the lateral dimensions of the beamrotator and the FAC lens are rotated substantially simultaneously. 15.The method of claim 9, wherein the beam rotator flips the fast-divergingaxis and the slow-diverging axis of each beam.
 16. The method of claim9, wherein the FAC lens is discrete and separate from the beam rotator.17. The method of claim 9, wherein the FAC lens and the beam rotator areportions of a unitary optical element.
 18. The method of claim 9,wherein the dispersive element comprises a diffraction grating.